https://developer.chrome.com/blog/cookie-max-age-expires/

This change does not impact session cookies—cookies that do not explicitly set an expiration date with Max-Age or Expires—as these are instead cleared when the browsing session ends.

Sound like either set by server in header or it session cookie.

Not found for firefox yet, maybe the same.

Edit: https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies

Permanent cookies are deleted after the date specified in the Expires attribute: or after the period specified in the Max-Age attribute:

Session cookies — cookies without a Max-age or Expires attribute – are deleted when the current session ends. The browser defines when the “current session” ends, and some browsers use session restoring when restarting. This can cause session cookies to last indefinitely.

Sound like firefox the same.